Electron energy spectrometer

ABSTRACT

A sample (2) is mounted in a sample holder (13) with a surface (3) of the sample (2) normal to the axis (4) of a pair of truncated electrically conductive frusto-cones (5, 6) which are coaxial and whose apexes meet at the sample surface (3). An exciting source (7) is mounted within the inner cone (5), which is solid and is maintained at ground potential to serve as a first electrode. The outer cone (6) is made of high transparency metallic mesh and is maintained at a positive potential +V (e.g. 1000 v) with respect to the sample surface (3), to serve as a second electrode. These components of the spectrometer (1) are contained within a vacuum system (15), and the potentials are applied to the cones (5, 6) by a biassing means (14). Electrons generated where the beam from the exciting source (7) strikes the sample are emitted into 2π steradians towards an entrance annulus (8). A small fraction of these electrons enter the entrance annulus (8) and find themselves in an electric field which deflects them towards the mesh of the outer cone (6). Electrons of a fixed kinetic energy leaving the sample (2) and entering the annulus (8) are accelerated towards the outer cone (6) on trajectories which will intersect. Those electrons that pass through the outer cone (6) enter a region of field-free space, in which their straight-line trajectories intersect on the surface of a third cone (11, FIG. 2), which is the focal locus of the spectrometer. As electrons of fixed kinetic energy enter the spectrometer through the annulus (8) between the cones (5, 6) they are focused into a ring on the focal locus.

This invention relates to electron spectrometers.

Electron spectrometers are in widespread use for the study of gases,liquids and solids in both academic and industrial contexts. Their mostwidespread use is in the characterisation and quantitative analysis ofthe surfaces of solids. In the semiconductor technology industry, theyare used to estimate the state of cleanliness of a surface before,during and after a large variety of different kinds of process stepsduring the production of integrated circuits. They are used also in thechemical industry to help establish manufacturing processes forcatalysts and polymers, and in the metallurgical industries to establishconditions for surface treatments for low friction coefficients, lowcorrosion in hostile ambient conditions and production of stronglyadhering coatings.

The dominant spectrometer in these areas is the coaxial mirror analyser(or CMA). This is a relatively simple instrument which consistsessentially of a pair of coaxial metal cylinders which are maintained atdifferent electrostatic potentials. A sample is mounted on the commonaxis of these cylinders and is bombarded by electrons or photons from asource which can be mounted within the inner cylinder. Electrons excitedby the photo-electric effect or the Auger effect leave the sample andenter the coaxial cylinders. If they have a kinetic energy appropriateto the dimensions of the structure and the voltages applied, then theyare focused onto an aperture and pass through to an electron multiplierwhere they are converted into an electrical signal which can be asanalog current or pulses which can be counted. The energy distributionof the electrons leaving the sample can be observe by sweeping thevoltage on the outer cylinder of the CMA so that electrons of varyingkinetic energies are detected at the electron multiplier. Only onenarrow range of kinetic energies is detected at one time and so thespectrum is swept sequentially.

This type of spectrometer has been very successful because:

(i) It is very simple in comparison to others and so it is relativelycheap to manufacture.

(ii) The exciting source is mounted inside the spectrometer itself whichleads to a compact structure which can be secured to an experimentalchamber by a single flange. This means that the whole assembly can beaccurately prealigned during manufacture so giving good control of theproperties of the entire instrument.

(iii) The single flange design allows other experimental apparatus to beadded with a straight line path from the sample to the additionalapparatus.

EP 0 470 478 discloses a type of CMA spectrometer.

A second kind of spectrometer, the concentric hemispherical analyser(CHA), has also become popular over the last few years. This is basedupon a pair of concentric hemispheres or sections of hemispheresaccessed by coaxial cylinder electrostatic lenses. This is a much morecomplex type of spectrometer, which may require anything between 5 and15 voltages to be varied as a spectrum is collected. However, it hasbetter energy resolution than the CMA, gives more space around thespecimen and can be configured electrically to operate in a variety ofmodes. The CHA has found favour in research and development laboratoriesbecause of this flexibility and the excellent energy resolution that ispossible. CHA instruments normally collect a single kinetic energy andat one time and so are swept sequentially like a CMA to collect anelectron spectrum. Since this kind of spectrometer is double focusing,it is possible to place an array of electron multipliers at its outputand collect several kinetic energies (or channels) simultaneously and sospeed up the acquisition of data. This was first done by placing ninechannel electron multipliers at the output of a CHA, so speeding upacquisition by a factor of nine. Subsequently, manufacturers haveincluded either separate multipliers (e.g. five) or micro-channel plateswith multiple collectors (e.g. sixteen) is order to improve the singlechannel restriction of the simplest form of CHA. However, it has notbeen possible to span more than about 50 eV of a spectrum at one time byadding such multiple detectors. This is because of the maximum energyrange presented by the hemispheres themselves at their output. Electronsof high kinetic energy strike the outer hemisphere and those of lowenergy strike the inner hemisphere and so both of these groups are lostto the detectors.

Because of the need to collect spectra sequentially, the total timerequired to collect a spectrum may be anything between a few seconds andseveral hours for an 8000 point spectrum, depending upon the energyresolution and the type of exciting source being used. Thus, fastexperiments must be confined to very narrow energy ranges and importantnew features cropping up in a spectrum can be missed. Further, wholeareas of application like single button operation for whole spectrumacquisition on a quality controlled production line are quiteimpossible.

In the publication "Phys. E: Sci. Instrum." by The Institute of Physics,Vol 13, 1980, pages 114-127, there is disclosed a coaxial coneelectrostatic velocity analyser, in which the cones have parallel walls.In the publication "Nuclear Instruments and Methods in PhysicsResearch", A298 (1990), pages 421-425, there are disclosed conicalanalysers with both parallel and diverging electrodes. In both of thesedisclosures, however, the principal of focusing the electrons followsthe established trend in the art. That is, the electrodes are opaque toelectrons, and formed with local slits through which electrons having anarrow band of energies pass, to be focused. They are essentiallytopological variations of the basic coaxial mirror analyser (CMA), andare incapable of focusing simultaneously electrons over a wide range ofenergies.

Preferred embodiments of the present invention aim to provide electronspectrometers which can be used to detect a wide energy rangesimultaneously, so that the entire electron spectrum over the importantand useful range from 20 to 2000 eV can be observed at once. Certainembodiments of the invention aim to achieve this, using a conventionalthermionic electron gus to excite the sample, in times of the order of50 msecs and less.

According to a first aspect of the present invention, there is provideda spectrometer comprising a sample holder, an excitation source, firstand second electrodes, biasing means and a detector, wherein:

the excitation source is arranged to emit an excitation beam to a samplein said holder thereby to cause electron emission from the sample;

the biasing means is arranged to establish an electric field betweensaid electrodes;

said electrodes are conical or part-conical in shape and are coaxialwith one another;

there is defined between adjacent ends of said electrodes a gap adjacentsaid sample holder to receive electrons emitted from a sample in theholder, in use;

said electrodes diverge from one another in a direction extending awayfrom said sample holder, and said detector diverges from said electrodesin a direction extending away from said sample holder, with the secondelectrode disposed between the first electrode and the detector; and

in a region where said electrodes diverge from one another and saiddetector diverges from said electrodes, the second electrode is at leastpartially transparent to electrons such that, in use, electrons enteringsaid electric field through said gap are deflected to pass through thesecond electrode and impinge upon the detector, which is operative todetect the impinging electrons.

Preferably, said electrodes are frusto-conical.

Preferably, the apexes of the respective cones of said electrodes meetat or adjacent a surface of a sample when held in said sample holder.

Preferably, the detector has a shape which is similar to that of saidelectrodes.

Preferably, the detector is coaxial with said electrodes.

A spectrometer as above, in accordance with the first aspect of theinvention, may further comprise a first screen which is disposed betweensaid second electrode and detector, and is arranged to be biassed to thesame potential as said second electrode.

Preferably, the first screen has a shape which is similar to that ofsaid detector and electrodes.

Preferably, the first screen is coaxial with said detector andelectrodes.

A spectrometer as above, in accordance with the first aspect of theinvention, may further comprise a second screen which is disposedbetween said first screen and detector, and is arranged to be biassed toa potential which is negative with respect to that of said first screen.

Preferably, the second screen has a shape which is similar to that ofsaid first screen, detector and electrodes.

Preferably, the second screen is coaxial with said first screen,detector and electrodes.

Said detector may include a light-emitting screen and means fordetecting said light.

Preferably, said detector includes an array of charge-coupled devices.

A spectrometer as above, according to the first aspect of the invention,may include processing means for receiving from said detector signalsrepresenting the distribution of electrons in said detector, and forprocessing said signals to provide a spectrum of the energy levels ofsaid electrons.

According to a second aspect of the present invention, there is providedan interface for use between first and second electric field regions,comprising a plate having first and second surface portions which bordersaid first and second regions respectively, at least said first surfaceportion comprising an electrically resistive material the resistance ofwhich varies over said surface portion so as to terminate and match oversaid surface portion equipotentials in the first electric field region.

Said second surface portion may comprise an electrically conductiveportion, to provide a termination where the electric field in saidsecond region is a zero field.

The invention extends to use of an interface according to the secondaspect of the invention to terminate equipotential sin at least oneelectric field region, comprising the steps of placing the interfacebetween first and second electric field regions, and terminating theequipotentials in said first electric field region by means of saidelectrically resistive material on said first surface portion, in such amanner as to match the potentials on said first surface portions withsaid equipotentials.

According to a third aspect of the present invention, there is providedelectron deflection apparatus comprising:

a pair of electrodes defining a space therebetween;

means for establishing an electric field in said space;

means for defining a gap between said electrodes, which means comprisesan interface according to the second aspect of the invention, the plateof which projects into said space to define said gap between one of saidelectrodes and an end of said plate; and

means for emitting electrons through said gap into said space.

Said plate may be disposed at adjacent ends of said electrodes.

Said apparatus may be a spectrometer--which may be as above, isaccordance with the first aspect of the invention.

The invention extends to use of a spectrometer according to any of theforegoing aspects of the invention to carry out a spectral analysis of asample, comprising the steps of:

holding the sample in the sample holder;

emitting an excitation beam from the excitation source to the sample insaid holder thereby to cause electron emission from the sample;

establishing an electric field between said electrodes by means of thebiasing means;

detecting, by means of the detector, electrons which enter said electricfield through said gap and are deflected to pass through the secondelectrode and impinge upon the detector; and

processing signals received from said detector to provide a spectrum ofthe energy levels of the electrons that have impinged upon sad detector.

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made, by way ofexample, to the accompanying diagrammatic drawings, in which:

FIG. 1 is a schematic longitudinal sectional view of the principalcomponents of one example of as electron spectrometer embodying thepresent invention;

FIG. 2 is an enlarged partial view of the spectrometer of FIG. 1,showing the upper part of the spectrometer above a longitudinal axis ofsymmetry, with additional components;

FIG. 3 is diagram illustrating the trajectories of electrons ofdiffering energies through a spectrometer of the type of FIG. 1 and 2;and

FIG. 4 is a detail view of an example of a conical entrance annulus ofthe spectrometer of FIGS. 1 and 2.

In the spectrometer 1 shown in FIG. 1, a sample 2 is mounted in a sampleholder 13 with a surface 3 of the sample 2 normal to the axis 4 of apair of truncated electrically conductive cones 5, 6 which are coaxialand whose apexes meet at the sample surface 3. (For convenience,frusto-cones such as 5, 6 may hereinafter be referred to simply ascones.) An exciting source 7, which may be, for example, as electron gunor a photon source, is mounted within the inner cone 5, which is solidand is maintained at ground potential to serve as a first electrode. Theouter cone 6 is made of high transparency metallic mesh and ismaintained at a positive potential +V (e.g. 1000 v) with respect to thesample surface 3, to serve as a second electrode.

These components of the spectrometer 1 are contained within a vacuumsystem 15, in a manner which is in itself generally well known in theart. The potentials are applied to the cones 5, 6 by a biassing means14, which may be located outside the vacuum system 15.

Electrons generated where the beam from the exciting source 7 strikesthe same are emitted into 2π steradians towards an entrance annulus 8 ofthe spectrometer, defined between the ends of the cones 5 and 6. A smallfraction of these electrons enter the entrance annulus 8 and findthemselves in an electric field which deflects them towards the mesh ofthe outer cone 6. Electrons of a fixed kinetic energy leaving the sample2 and entering the annulus 8 are accelerated towards the outer cone 6 ontrajectories which will intersect. Those electrons that pass through theouter cone 6 enter a region of field-free space, in which theirstraight-line trajectories intersect on the surface of a third cone,which is the focal locus of the spectrometer. A detector assembly can beplaced on this focal locus. As the electrons of fixed kinetic energyenter the spectrometer through the annulus 8 between the cones 5, 6 theyare focused into a ring on the focal locus.

A simple detector which can be placed at the focal locus is afluorescent phosphor screen which can be viewed through a vacuum windowby a closed circuit TV camera. The electrons reaching this screen areaccelerated after they have passed through the field-free space wherethey are focused, in order that fluorescence in the phosphor of thescreen can be excited.

Such a simple version of the spectrometer 1 is sketched in FIG. 2, wherethere are now two extra conical grids 9, 10 between the outer cone 6 andthe fluorescent screen 11. The grid 9 is at the same potential as theouter cone 6 (e.g. +1000 V) and it ensures that the electrons move infield-free space having left the outer cone 6. The grid 10 is placed ata potential (e.g. +500 V) which is negative with respect to the grid 9,and forms a high-pass filter between the grid 9 and the fluorescentscreen 11, which is maintained at about 5 kV. The grid 10 is desirablebecause those electrons which happen to strike the metal of the outercone 6 will cause secondary electron emission. These secondaries couldreach the fluorescent screen 11 and give an unwanted background to thespectrum and so they should be rejected. By placing the grid 10 at apotential which is negative with resect to the outer cone 6, thesecondaries will be rejected by turning them around before they reachthe screen 11.

Thus, the whole spectrometer 1 consists, in this example, of the sample2 (in its holder), the excitation source 7 and the assembly of the fivecoaxial frusto-conical components 5, 6, 9, 10 and 11, all containedwithin a vacuum system. In the first instance, detection can be carriedout using, for instance, a TV camera outside the vacuum system viewingthe fluorescent screen through a vacuum window. The whole assembly canbe mounted like a CMA. However, unlike a CMA, the whole spectrum can bemade to appear on the fluorescent screen 11 at once, and it can beconverted to electrical form in a single TV frame scan time.

Further practical aspects of the above-described spectrometer 1 may beas follows.

The outer cone 6 may be made of stainless steel woven mesh of high (e.g.80%) transparency.

The secondary electron generation processes at the mesh of the outercone 6 should not be allowed to interfere significantly with the energyanalysed electrons being focused by the spectrometer on the screen 11.The grids 9 and 10 form the high-pass filter which rejects thesesecondaries. The arrangement of these grids requires careful design toensure that they are close to the detection plane of the screen 11, andyet do not suffer from field electron emission. The formed grids may beelectropolished and then coated with gold to provide a smooth surface ofconstant work function.

The secondary electron generation processes at the end of the cones 5, 6remote from the sample 2 have to be considered carefully because theequipotentials must be terminated here. This depends very much on thechoice of the ratio of the overall length of the cones 5, 6 to theuseful length through which energy analysed electrons will pass. Thegreater this ratio, the smaller the fraction of secondaries will reachthe screen 11.

An important component is a tapered resistive film entrance aperture 40which matches field-free space to the conical equipotentials inside thecones 5, 6--as is illustrated in FIG. 4 and described later in moredetail below.

A TV camera may be provided to collect the light from the fluorescentscreen 11, and may be interfaced to a computer to extract spectra bycircular averaging of the image. Such cameras, respective control boardsand device handling software are all available commercially at thepresent time.

It is not necessary, however, to convert electrons to light and thenback to electrons for display, storage and processing. Thus, instead ofthe fluorescent screen 111 which is monitored by a TV camera undercomputer control, there may alternatively be employed an integrateddetector array--that is, a device to detect charge directs at the focallocus of the spectrometer. This may reduce sensitivity to ambientlighting levels, allow the realisation of the shot-noise limitedstatistics in the electron detection, and facilitate simple directinterfacing to a control computer. A variety of detectors may beconsidered for this.

A conventional solution would be to use a pair of channel plates and anarray of metallic strips behind them to collect the amplified charge.However, this may be very difficult in practice, because a conical focallocus would require the development of a conical microchannel plateassembly. A special case of the general conical geometry is possiblewith a plane circular focal locus. Such a special design could be usedwith commercial available microchannel plate assemblies. In both casesthe output strips would have be brought individually through the vacuumwall. Bringing 1000-8000 separate high speed, low signal level leadsthrough a UHV wall is not an easy proposition.

Another approach would be to use a pair of conical or planarmicro-channel plates followed by a resistively encoded positionsensitive detector. This would bring the number of output leads down to4 but is unlikely to have the dynamic range required to resolve as manyas 1000 channels. It would still require the development of the conicalchannel plate assembly.

A preferred approach is to use an integrated array of charge-coupleddevices (CCD's) together with a fast multiplexer. Chips for thisdetector might be fabricated as flat triangular shapes which would bemounted and interconnected as a frusto-conical assembly to replace thescreen 11.

A detailed description of the sample holder 13 is not essential to anunderstanding of the invention. The purpose of the sample holder 13 isto hold the sample 2 to be analyzed in a desired location. This willusually be at the apexes of the conical electrodes 5, 6 (or where theirapexes would meet, if they were not frusto-cones), although the samplemay be disposed at other locations if desired--usually on the axis 4,but possibly to either side of it, and/or to the right or to the left ofthe position as seen in FIG. 1.

Thus, many possible forms of the sample holder 13 will be apparent tothe worker skilled in the art. As will also be understood by the workerskilled in the art, the term "sample holder" may include means forpresenting any sample to be analysed at the desired location. Forexample, if a gas is to be analysed, the "sample holder" may comprise agas flow cell, to present a flowing stream of gas to be analysed at theapexes of the cones 5, 6 (or other desired location).

Although the cones 5, 6 are preferably frusto-cones, to enable thesample 2 to be placed at their coincident apexes, they may alternativelybe full cones, in which case the sample would be placed to the right ofthe position as seen in FIG. 1.

A more detailed theoretical consideration of the spectrometer 1, andmodifications thereof, will now be given.

Consider a pair of metallic cones (such as the cones 5, 6) with theircommon apexes at the source of electrons. The inner and outer cones aremaintained at V₁ and V₂ and have cone semi-angles θ₁ and θ₂respectively. Laplace's equation is soluble in spherical polarcoordinates for this configuration and so equations for theelectrostatic potential V(r, θ) and the electric field E(r, θ) can befound. They are: ##EQU1##

In these equations, the quantity Z is related to the polar angle θ by:##EQU2##

Z₁ and Z₂ are thus the values of Z at the cone surfaces where θ is θ₁ orθ₂. It can be seen that V is independent of r and so the equipotentialsbetween the cones are themselves conical surfaces. The magnitude of thefield E is such that is falls off as l/r away from the electron sourceand it is circumferential in sense.

The trajectories of electrons leaving the sample 2 can be calculated ifit is assumed that they travel in straight lines towards the cones 5, 6which start at a distance r₀ from the origin where the analytic field inEquation (2) turns on abruptly. They then move in curved trajectoriestowards the most positive cone. If the outer cone 6 is the mostpositive, as shown in FIG. 2, then the coordinates of the point where aray of given kinetic energy cuts the outer cone 6 can be calculatedanalytically. The electron then moves in a straight line in thefield-free space between the outer cone 6 and the inner grid 9. Atypical electron trajectory 12 is shown in FIG. 2.

If an annular cone of electron trajectories is admitted to the volumebetween the cones 5, 6, then the straight line paths outside the outercone 6 do not cross at a single point. However, they do cross within anarrow region (the focal point for those rays), the width and positionof which can be found by numerical least squares analysis of a set ofpaths within the entrance annulus.

We have done this by means of a computer program, and found the surfacejoining the focal points for a set of kinetic energies of electronsentering the spectrometer 1. We have also plotted the trajectories ofthe electrons through the cones and the foal locus. We have used anentrance annulus defined by the suer and containing 21 beams launched atdifferent directions into the spectrometer within this annulus. Thekinetic energy range of electrons within the annulus can be divided intoup to 50 discrete energies so that the energy dispersion and resolutioncan be examined in detail.

Using this program, the focusing properties of the spectrometer havebeen examined as a function of the angle between the cones, thesemi-angle of the inner cone, the cone lengths and the distance r₀between the sample and the start of the cones. An important objective inthis exploration has been to find a focal locus which is as near to aplane as possible in order to simplify the fabrication of the detector.Further, solutions were sought which did not cause the electrons withlow kinetic energies to focus to the left of the sample surface 3 asdrawn in FIG. 2. This is because it was wished that the structure of thespectrometer does not obstruct access to the sample 2. It was discoveredthat the best resolution was always obtained if the entrance annulus wasdefined so as to cause electrons to enter the volume between the conesvery near to the inner cone.

A spectrometer with good resolution, a focal locus always to the rightof the sample surface 3 and a nearly flat conical focal locus as foundto have the specification given in Table 1.

                  TABLE 1                                                         ______________________________________                                        SPECTROMETER SPECIFICATION                                                    ______________________________________                                        Configuration                                                                 Inner Cone semi-angle, θ.sub.1                                                                 53 deg                                                 Outer Cone semi-angle, θ.sub.2                                                                 67 deg                                                 Inner Cone potential, V.sub.1                                                                        0 volts                                                Outer Cone potential, V.sub.2                                                                        1000 volts                                             Overall cone length, L 60 mm                                                  Clear radius to spectrometer, r.sub.θ                                                          10 mm                                                  Entrance annulus       54-56 deg                                              Properties                                                                    Resolution             1003                                                   Solid angle collected  2.9% of 2π sr                                       Focal plane            y = 4.9x + 11                                          Mean Dispersion        16 μm eV.sup.-1                                     ______________________________________                                    

A diagram of the trajectories of eleven electron beams from 50 eV to2050 eV, through the spectrometer of Table 1, is shown in FIG. 3, whichshows that the focal locus is a cone containing the straight line withpositive slope passing through approximately (0, 11).

The spectrometer is an approximately constant resolving power device inthat the diameter of the focus increases linearly with the kineticenergy of the electrons being passed. Also, the radial distance alongthe focal plane of the focal position increases approximately linearlywith kinetic energy. These effects mean that the intensity of thefluorescent screen 11 at the focal plane will be proportional to EN(E)where N(E) is the energy distribution of the electrons leaving thesample 2. This is similar to the nature of a spectrum detected using aCMA.

Table 2 below is similar to Table 1, but shows another spectrometer withan even flatter focal locus.

                  TABLE 2                                                         ______________________________________                                        SPECTROMETER SPECIFICATION                                                    ______________________________________                                        Configuration                                                                 Inner Cone semi-angle, θ.sub.1                                                                 67 deg                                                 Outer Cone semi-angle, θ.sub.2                                                                 79 deg                                                 Inner Cone potential, V.sub.1                                                                        0 volts                                                Outer Cone potential, V.sub.2                                                                        1000 volts                                             Overall cone length, L 90 mm                                                  Clear radius to spectrometer, r.sub.θ                                                          20 mm                                                  Entrance annulus       68-69.5 deg                                            Properties                                                                    Resolution             1335                                                   Solid angle collected  2.4% of 2π sr                                       Focal plane            y = 267.5 + 69.2x                                      Mean Dispersion        23 μm eV.sup.-1                                     ______________________________________                                    

Realisation of the attractive design of the spectrometer of Table 1 or 2requires that the field-free region between the sample 2 and theentrance annulus 8 to the cones 5, 6 be matched to the conicalequipotentials inside the cones 5, 6. In addition, the entrance annulus8 has to be defined to give the appropriate annular cone angle.Inspection of Equation (3) reveals that the logarithmic cosine functionZ is what determines the variation of the potential in the θ direction.A plot of this function for the spectrometer of Table 1 would show thatthe potential varies very nearly linearly with θ in the range 50°≦θ≦70°.Clearly, a metallic aperture closing the front ends of the cones 5, 6(except for the entrance annulus 8) would not terminate theequipotentials correctly and the low kinetic energy electrons would moveon paths very from the analytic case described above. Indeed, aninvestigation of the effects of apertures has shown the distortion ofthe equipotentials near a grounded simple metallic aperture to beclearly unacceptable.

A novel alternative structure for the entrance annulus 8 may be providedby an insulating sheet coated with a film of a good conductor (goldwould be suitable) on the side of the sample and a thin resistive filmon the side facing the insides of the cones 5, 6. Such an aperture plate40 is shown in FIG. 4.

In FIG. 4, a thin glass cone 41 is formed on a carbon former. Aftercleaning, the outer surface of the cone 41 is coated with a taperingsilicon film 42 of about 10⁹ ohms resistance by vacuum evaporationthrough the adjustable iris. Control of the iris provides the desiredthickness profile. The inside of the cone 41 and the outer wall 43 ofthe entrance annulus 8 are coated with a high conductivity gold film 44which, in use, is grounded on the side facing the sample 2, so allowingelectrons to move in field-free space from the sample 2 to the entranceannulus 8.

If a resistive film of uniform thickness were evaporated on the outsideof the conical aperture plate 41 of FIG. 4, it would have a potentialdistribution along its surface given by: ##EQU3##

This is clearly not linear as needed. A good approximation to thespacing of the equipotentials inside the cones can be achieved bytapering the thickness of the annular resistive film 42 in such a waythat its thickness varies as the reciprocal of the distance between theouter wall 43 of the entrance aperture 8 and the inner surface of theouter cone 6. In this case, the potential along the surface of theresistive film 42 becomes: ##EQU4##

In equation (5), the distance R is simply the radial distance from theaxis of the conical aperture plate 41 to the point where the potentialis being measured.

A finite element calculation for this tapered resistive film apertureshows that the disturbance of the equipotentials is not very muchsmaller and is acceptable.

If a resistive film aperture of this design is to be used in practice,then one important consideration is the power dissipated in the aperture(it is connected across 1000 volts in the spectrometer 1) and the powerdissipated in the voltage supply providing the cone potentials. Torestrict the power consumption to 1 mW and so have negligible apertureheating, the tapered film needs to go from 110 nm thick at the outercone 6 to 90 nm thick near the inner cone 5 and have a resistivity ofabout 25 ohm.cm. This may be realised if the silicon film 42 is ofpolycrystalline silicon evaporated onto the aperture plate 41.

An estimate of the current reaching the focal plane can be made asfollows. Consider excitation by a 5 keV beam of electrons and a beamcurrent of 1 μA. If the secondary electron yield of the sample is 1(numbers between 0.8 and 5 occur in practice) and the analyser accepts2.8% of 2π or then a total current of 2.8×10⁻⁸ A enters the cones 5, 6.If the energy distribution of the secondary electrons is approximated asbeing flat from zero eV to the primary energy, then the current densityis 5.6×10⁻¹² A per eV. The spectrometer 1 has an average energy windowof 2 eV and so the means current detected at the fluorescent screen 11or the alternative integrated detector will be about 10⁻¹¹ A for eachenergy channel.

This is sufficient to excite visible fluorescence in a screen (electronmicroscopes often work with 10⁻¹² A) and so should give a measurableintensity for a TV camera. For an integrated detector this correspondsto an arrival rate of about 6×10⁷ electrons per second or a charge ofabout 10⁻¹³ C in a 10 msec data acquisition time. Such a charge iseasily detectable with a CCD device.

Thus the results of the above analysis show that a spectrometer such asthe spectrometer 1, with a resistive film aperture such as that shown inFIG. 4, may collect 3% of 2π steradians for analysis and separateelectrons with kinetic energies between 50 and 2050 eV into 1000channels with an energy resolution of about 2 eV per channel. Thiscompares very favourably with both CMA and CHA known spectrometers.Thus, a CMA may typically collect 10% of 2π steradians but only 1channel. A CHA may collect 2% of 2π steradians and only 16 channels atthe best. If the dwell time per channel is 10 msecs (a realisticpractical figure) then a CHA may be 23.2 times faster than a CMA, but aspectrometer such as the spectrometer 1 may be 330 times faster.

Other comparisons are possible. For example, a spectrometer such as thespectrometer 1 but collecting 0.7% of 2π steradians may haveapproximately a 0.1 eV energy resolution whilst collecting 8000 channelsbetween 50 and 2050 eV simultaneously. This is very significantly betterthan either a CMA or a CHA and would be an extremely useful instrumentin a wide variety of applications.

In the illustrated embodiments of the invention, the conically shapedelectrodes, girds and screens may be full cones (or frusto-cones), inthe sense that they subtend a full 360°. however, in alternativeembodiments, they may subtend less than 360°. For example, they may behalf-cones (or frusto-cones) subtending 180°, quarter cones (orfrusto-cones) subtending 90°, or any other fraction of full 360° cones(or frusto-cones). This may facilitate access to components of thespectrometer.

It is important that the cone 6 is at least partially transparent toelectrons, so that they may pass through the cone 7 to impinge upon thedetector. For the avoidance of doubt, the term "at least partiallytransparent" means that any given area of the transparent material willallow a significant proportion of electrons reaching the electrode topass through it--as opposed to an opaque material which is substantiallyimpervious to electrons, but which is formed with one or more smalllocal aperture (e.g. a slit) to act as a mask, and allow electrons topass freely through only that aperture.

In the foregoing examples, the cone may be of a very fine mesh having ahigh degree of transparency to electrons--e.g. about 80%. As will beunderstood by the worker skilled in the art, the wires of fibres of themesh will tend to collect electrons that collide with them, and thusprovide the smaller degree of opacity (e.g. about 20%) of the mesh.

the cone 6 may be of alternative materials--e.g. complex solids whichhave an intrinsic degree of transparency to electrons. Usually, thetransparency of the material will be uniform over the full area of thecone 6--although certain areas (e.g. at supports) may be locally moreopaque or fully opaque. It is possible also for areas of the cone 6 tobe more or fully opaque where no electron transmission is expected ordesired. The main thing is to allow a sufficiently large area oftransparency to allow electrons over a wide band of energies (preferablyall electrons energies that may be expected to be emitted in thespectrometer) to pass through the cone 6--as opposed to, for example,previously proposed spectrometers which allow only one or more narrowranges of electrons to be focused at nay one time.

Preferably, the cone 6 has a transparency of at least 50% to electrons,over areas where electrons may be expected to meet the cone 6.

Advantages arise from having the second cone 6 outside the first cone 5.For example, the size of the focal locus where electrons are detectedincreases with distance from the axis 4. As mentioned above, at oneextreme, the focal locus could be a plane--in which case the detectorcould have the form of a flat disc (an extreme cone with cone angle of180°). Indeed, in this case, the detector could be of any shape--evennon-symmetrical and/or non-aligned with the axis 4, provided that itwere plane and positioned at the focal plane to detect at least part(preferably all) of the electrons focused there. Also, with the secondcone 6 outside the first cone 5, connections between the detector andperipheral/ancillary components may be easier.

However, it is possible alternatively to dispose the second(transparent) cone 6 within the first cone 5, with the detector thenwithin the second cone 6. This may provide further protection for thedetector, but the focal locus will tend to be smaller, and connectionsto the detector may be more difficult. In the extreme case, the focallocus of the detector may be a circular cylinder (an extreme cone withcone angle of 0°).

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed in one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or nay novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

I claim:
 1. A spectrometer comprising a sample holder, an excitationsource, first and second electrodes, biassing means and a detector,wherein:the excitation source is arranged to emit an excitation beam toa sample in said holder thereby to cause electron emission from thesample; the biassing means is arranged to establish an electric fieldbetween said electrodes; said electrodes are conical or part-conical inshape and are coaxial with one another; there is defined betweenadjacent ends of said electrodes a gap adjacent said sample holder toreceive electrons emitted from a sample in the holder, in use; saidelectrodes diverge from one another in a direction extending away fromsaid sample holder, and said detector diverges from said electrodes in adirection extending away from said sample holder, with the secondelectrode disposed between the first electrode and the detector; and ina region where said electrodes diverge from one another and saiddetector diverges from said electrodes, the second electrode is at leastpartially transparent to electrons such that, in use, electrons enteringsaid electric field through said gap are deflected to pass through thesecond electrode and impinge upon the detector, which is operative todetect the impinging electrons.
 2. A spectrometer according to claim 1,wherein said electrodes are frusto-conical.
 3. A spectrometer accordingto claim 1, wherein the apexes of the respective cones of saidelectrodes meet at or adjacent a surface of a sample when held in saidsample holder.
 4. A spectrometer according to claim 1, wherein thedetector has a shape which is similar to that of said electrodes.
 5. Aspectrometer according to claim 1, wherein the detector is coaxial withsaid electrodes.
 6. A spectrometer according to claim 1, furthercomprising a first screen which is disposed between said secondelectrode and detector, and is arranged to be biassed to the samepotential as said second electrode.
 7. A spectrometer according to claim6, wherein the first screen has a shape which is similar to that of saiddetector and electrodes.
 8. A spectrometer according to claim 6, whereinthe first screen is coaxial with said detector and electrodes.
 9. Aspectrometer according to claim 6, further comprising a second screenwhich is disposed between said first screen and detector, and isarranged to be biassed to a potential which is negative with respect tothat of said first screen.
 10. A spectrometer according to claim 9,wherein the second screen has a shape which is similar to that of saidfirst screen, detector and electrodes.
 11. A spectrometer according toclaim 9, wherein the second screen is coaxial with said first screen,detector and electrodes.
 12. A spectrometer according to claim 1,wherein said detector includes a light-emitting screen and means fordetecting said light.
 13. A spectrometer according to claim 1, whereinsaid detector includes an array of charge-coupled devices.
 14. Aspectrometer according to claim 1, including processing means forreceiving from said detector signals representing the distribution ofelectrons in said detector, and for processing said signals to provide aspectrum of the energy levels of said electrons.
 15. Use of aspectrometer according to claim 1 to carry out a spectral analysis of asample, comprising the steps of:holding the sample in the sample holder;emitting an excitation beam from the excitation source to the sample insaid holder thereby to cause electron emission from the sample;establishing an electric field between said electrodes by means of thebiassing means; detecting, by means of the detector, electrons whichenter said electric field through said gap and are deflected to passthrough the second electrode and impinge upon the detector; andprocessing signals received from said detector to provide a spectrum ofthe energy levels of the electrons that have impinged upon saiddetector.
 16. A spectrometer according to claim 1, comprising aninterface for defining said gap between said electrodes, which interfacecompanies a plate having first and second surface portions which borderfirst and second electric field regions respectively, at least saidfirst surface portion comprising an electrically resistive material theresistance of which varies over said surface portion so as to terminateand match over said surface portion equipotentials in the first electricfield region.
 17. An interface according to claim 16, wherein saidsecond surface portion comprises an electrically conductive portion, toprovide a termination where the electric field in said second region isa zero field.
 18. Apparatus according to claim 17, wherein said plate isdisposed at adjacent ends of said electrodes.